Should I fit a transmission oil cooler for heavy towing | Auto Expert John Cadogan

Towing a heavy trailer, presumably, in Hell: Do you really need to add that expensive transmission oil cooler?

As an engineer, the way I look at this is: Have you measured the transmission oil temperature, or other operating temperatures, and drawn some rational conclusion that something needs to be modified? Or are you just guessing?

Are we using evidence? Have we plugged in some Bluetooth dongle and become concerned because the transmission oil temperature during one of these Top End caravanning ‘hell on earth’ incursions is higher than the maximum permissible oil operating temperature limit in the oil specs?

Alternatively, is there evidence, perhaps, that on your make and model of 4X4, that there are premature failures of the transmission stemming from over-temp operation while towing in hot climates?

I’d suggest a lot of men spend their downtime streaming what amounts to ‘modification porn’. Hey - it’s a victimless crime. I don’t even think you go to hell for that. But you might wake up in automotive hell if you actually make unnecessary modifications - especially on a brand-new car.

You’ll be essentially devoting a lot of time, effort and money to ‘solving’ (if that’s the right word) a ‘problem’ (if that’s the right word) that most likely does not exist. If that’s the case, it’s a completely redundant activity.

In addition to wasting money and time, redundant modifications are a great way to increase complexity (which potentially reduces reliability - because those two things are inevitably conflicted), and it can introduce malicious feedback effects, causing premature failures.

Which is exactly what you are trying to avoid.

If that happens, it can easily void what’s left of your factory warranty - especially if a catastrophic failure of the cooler or bypass valve dumps your transmission’s lifeblood all over the road somewhere between Outback Shithole A and Outback Shithole B, after which there is generally a loud noise and deafening silence … followed by a protracted delay, and a five-figure repair bill.

Just picture it - all that R&D done at the factory, getting vulnerable components out of the way of - whatever - pursued with great zeal by engineers who take it all quite seriously, then a stone flicks up and hits that expensive aftermarket solenoid valve … no more oil … no more transmission. No more outback fun.

Unless there’s actual evidence, therefore, give the engineers some credit for enduring that extreme hot weather tow testing in R&D. Generally, the factory will recommend fitting an oil cooler with the tow kit if a transmission durability problem comes to light in R&D.

If there is evidence, go for it. Fit the cooler. Fit the factory one if such a unit exists. If not, do it ‘overkill’. Use the best cooler, best hardware and best tradesman you can find. Don’t scrimp. Don’t DIY it unless you have trade-level competency in this domain.

I’d also suggest a lot of this has to do with how close to the ATM and GCM limits you are when you’re towing. (That’s ‘aggregate trailer mass’ - the most heavily laden weight of the trailer, and ‘gross combination mass’ - the all up weight of the loaded trailer and loaded vehicle combined).

Cruising speed matters, too - because you’re pushing a hell of a lot of air out of the way, and that consumes prodigious energy - and generates a lot of heat. Small reductions in cruising speed make a big difference to aerodynamic drag - so you might want to knock it back 10 kays when the mercury hits 40.

Operating at a really high ambient temperature reduces the convective heat loss capacity of the vehicle’s various cooling systems, obviously. But manufacturers tend to be A) very cruel to prototypes in R&D, and B) very conservative on limits and cooling capacities by the time the vehicle finally makes it into production.

So - you know how it is with doctors, right? The Hippocratic oath? First, do no harm. In my view that’s not a bad way to approach modifications. And of course, don’t treat the patient at all if it’s not actually sick.

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